If the containers are thermoformed directly in a sheet of thermoplastic material significant waste zones are created between these containers on account of the space taken up by the thermoforming moulds and counter-moulds. Document FR 2 827 258, to limit material losses corresponding to these waste zones, recommends stretching said zones before thermoforming. This method is efficient but it requires relatively complex kinematics.
It is also known, for thermoforming, to use pre-cut out blanks. In this case the blanks are placed on a conveyor and conveyed by the latter to a heating station and then to a thermoforming station. However, it is difficult to hold blanks in place correctly whilst fully heating the entirety of their lower and upper surfaces. In general, the supporting thereof does not only affect the edges of the blanks but also affects at least the lower surfaces of the blanks. Document FR 2 842 136 examines this problem and recommends using blank conveying means comprising cells which leave clear the entirety of the upper and lower surfaces. However these cells must be associated with supporting means that are relatively complex to implement.
In addition, document FR 1 554 475 discloses the conveying of a sheet of thermoplastic material to the input of a thermoforming installation, cutting blanks from this sheet and then transferring the blanks to a heating station, and finally transferring the hot blanks to a thermoforming station. The difficulty of heating the entirety of the lower and upper surfaces of the blanks still fully remains in this document.
Also, document JP 60-78422 discloses heating a sheet of thermoplastic material, cutting blanks from this hot sheet and conveying the blanks to a thermoforming station for thermoforming of the containers. Therefore the blanks are perfectly heated in their entirety before thermoforming is carried out. However, this document does not seek to limit material losses in the sheet of thermoplastic material.
Document U.S. Pat. No. 4,436,685 discloses an installation in which thermoforming is carried out by continuous extrusion of a sheet of thermoplastic material. Sections of sheet are cut and reheated then transferred to a thermoforming station for the thermoforming of containers in these sections.
Similarly, this document does not seek to limit losses of material in the sheet or sheets sections of thermoplastic material.